Peavey Butcher 6l6 to El84

[sonic_tooth]

Sorry about the double post, I could not figure out how to remove earlier my post from the mod/tweak forum. Could one moderators please remove it. Thanks...

I am trying to mod my Peavey Butcher to use EL34s. People seem to indicate that this is a pretty easy mod, but there does not seem to be much documentation on it. I am a beginner so my knowledge is somewhat limited. I was able to find the info below to use as a guideline for the mod.

Stock Schematic
http://www.schematicheaven.com/newam...ey_butcher.pdf


Mods
"Phase Inverter- change R24 and R27 to 1 Meg, R23 to 82K, R26 to 100K.
Bias- remove R53(47K) and replace with 2 wires to a 10K Pot(one end of pot to R52 hole, the wiper to hole that goes to PC board conector. Then attach a 15K to other end of pot to the hole that goes to ground.)
Power tubes-Break (cut with a dremel...) lands on PC board between pins 1 and 3, then join pins 1 and 8 with a wire.
Change R3,R5,R7,R9 from 100/5 watt to 1K/5 watt.
You should have about 515 volts on pin 3 and the bias will adjust from -53 to -31. "

After checking the schematic it does not seem like cutting the traces on the pins would work because if in between 1 and 3 was cut I would still be sending 508+ to ground after joining it to pin 8. So I disconnected pin one from the board completely and tied it to pin 8. I also disconnected pin 8 and put in 1 ohm resistor in between it and the board for cathode resistor biasing. I performed the PI mods and upped the screen resistors as listed. For my bias control I just went with a 50 k trimmer.

Now when I power on my amp none of my tubes (power or pre) light up at all. I am getting 508+ on pin 3 and -55 on pin 5 of my power tubes. I am not getting any voltage across my 1 ohm cathode resistors as well. I am not sure if the mods I did are incorrect or if I damaged something else while I was performing it. Anybody have any ideas?

[Gtr_tech]

Pins 1 and 6 aren't used on a 6L6 so they may have used them as tie points. On an EL34 pin 1 is the suppressor which needs to be grounded. If pins 1 and 3 are tied together you'd have to cut that trace (make it a wide cut so it won't arc) and connect pin 1 to gnd. You're not reading any cathode current 'cause the heaters aren't working....so nothing is working.

Now....will your pwr xfmr handle the extra load from the El34s? 6Ls draw 900mA, while 34s draw 1.5A. Thats damn near double and asking a lot of the existing filament winding. I'd say put them in and with the amp on stby measure the ac heater voltage. It shouldn't be much lower than 6.3vac. If it is you're overloading that winding and will burn it out in no time. And if you let it sit in stby for a while does the pwr xfmr get too warm, to touch? If so ,bad mojo. Turn back now.

My advise? Stick with good quality 6Ls. If you want an EL34 amp get one thats designed to run them.

As far as the heaters not working, its either a fuse or the molex header connectors. Best thing to do to those headers is to hardwire the heater lines. Just jump them with some 20awg solid wire under the boards from connector to connector. They frequently break loose from the solder connections and flame up the boards.

[Enzo]

After checking the schematic it does not seem like cutting the traces on the pins would work because if in between 1 and 3 was cut I would still be sending 508+ to ground after joining it to pin 8. So I disconnected pin one from the board completely and tied it to pin 8

As Gtr said, pin 1 is not used on a 6L6. So they ran the trace to pin 3 right through it instead of wedging between traces. There is nothing wrong with lifting the pin 1 and shunting it to 8. The idea in cutting traces is that you cut to isolate pin 1 from the traces going to the pins 3. The only way there would be 500v on pin 1 is if you didn't cut the trace feeding pin 3 away from pin 1. If the trace with 500v is coming from somewhere on its way to pin 3 and gets to pin 1 first, yes, ther would be 500v there, but that would be only doing half the job if you cut merely the remainder of the trace between the two pins only.

[sonic_tooth]

Thanks guys it was a bad fuse in the power section. Amp fires up and I get sound out of it. My bias control (R53) does not seem to be working though. The power transformer seems to be handling the EL34s just fine (it is not getting hot at all). I am only showing 3ma across my cathode resistors.

[Enzo]

PV transformers aer usually very sturdy, so the extras heater current shouldn't bother it.

Your bias control doesn't work because you didn't change R52. The mod described is wrong. They have you replacing the 47k R53 with 25k (10k pot plus 15k resistor.) That pot and resistor would work in something like a 5150, but that has a different circuit. With R52 only 470 ohms, it will be very difficult to do anything to R53 and expect it to control the bias voltage.

R52 and R53 form a voltage divider across the -57VDC supply on C27. Calculate the bias voltage thus:

(R53)/(R53 + R52) x -57v = bias volts.

If R52 is really low, then it doesn't do anything to the equation.

REplace R52 with something like 3k to 5k. 3.3k is a standard value, nothing cosmic about 3k exactly. Now when we vary R53, we have room for the ratio to shift. Replace teh 47k with a pot and resistor that add up to about the same. FOr example 27k resistor and 25k pot. Again it is not cosmic. In that example then the total value of the R53 spot is 27k - 52k.

[sonic_tooth]

Enzo, thanks for the help the bias control is working great. Man this thing is a real fire breather now.... Anyway seems like the problem with the flakey filament connectors is still an issue... must not have been the fuse I replaced. I will have to get rid of those connectors and run some jumpers like Gtr_tech suggested. Thanks again!!

[sonic_tooth]

Well, the amp seemed OK for a while, but now it seems to be running at 1/2 volume. I thought I had fixed the filament issues, but while I was running the amp it the filaments went out and came back on and now the amp is running at 1/2 of the volume that it was. What voltage should I be seeing on the heaters? I can't seem to find a flake. What could this be?

[sonic_tooth]

http://www.schematicheaven.com/newam...ey_butcher.pdf

Sorry, I just realized my previous schematic liink does not work, here is a good one.

[sonic_tooth]

I wired all of the filament connections directly to the board. The output is still about half of what it should be. It seems like this has to be some kind of supply problem. The tubes (pre and power) do not seem to be lighting up as brightly as the should. I have gone through the supply board checking for bad joints it seems to be ok. The only thing that I think is suspect are the other molex connectors on the supply board. Is hardwiring all of the connections on the supply board a good next step?

[Enzo]

I would say stop guessing and actually find the problem. 6v tubes should have about 6.3VAC across their heaters. Unless you definitely can see that all the heaters are definitely all dimmer than they used to be, don't rely on visual inspection of the heaters. Usually if you see them glow, the 6v is OK. COnsider the age of this amp, and it is just now starting to work again.

Plug a cord from FX send to FX return. If it restores the sound, the return jack needs services. If there is a preamp out and power amp in pair of jacks, try the same thing there.

DOn't GUESS your voltages are low, FIND OUT. If you think the heaters are weak, MEASURE the voltage across a tube heater right at its socket. Not from each pin to ground, but right from one end of the heater to the other. if you think the B+ is low, MEASURE it.

If the heaters are coming and going, the transformer wires go to the power supply board, through a fuse and back off that board. Both the entering and leaving connections for that are suspect. Then the wires go to the power tube board and the preamp board. Check those connections. If ALL the tubes come and go at once, then suspect the connections on the power supply. If preamp tubes go and power amps stay or vice versa, then suspect the wires between the boards.

[sonic_tooth]

The filaments do not seem to be intermittent anymore and I am showing 6v across them. My B+ is 460v, a little low, but I don't think it is the culprit. There is no effects loop or power amp in, only a line level send.

[sonic_tooth]

Well now I am not sure about my B+ voltage. I turned on my calculator and checked the stored values and it looks like I was origianlly getting around 490v instead of the 460v that is showing up now. Would a 30v difference really account for that big of a difference in volume?